Device and method for computer tomography

ABSTRACT

A device and a method for the multispectral correction of radiation hardening in computer tomography with variable tube voltage is described. In particular, a water correction and a post-reconstructive hardening correction is disclosed. To perform the water correction, project image data is corrected, in that correction values are obtained from a previously determined correction table, by means of which a correction of the projection image data can be performed. By means of an image reconstruction, this produces a corrected volumetric image.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German application No. 10 2005 028216.4 filed Jun. 17, 2005, which is incorporated by reference herein inits entirety.

FIELD OF THE INVENTION

The invention relates to a device for computer tomography with:

-   a radiation source for x-raying an object to be examined from    various projection directions,-   a detector for detecting the radiation from the radiation source and-   an evaluation unit downstream of the detector that corrects, with    respect to radiation hardening, the projection images, taken by the    detector, of an object to be investigated.    The invention also relates to a method for computer tomography.

BACKGROUND OF THE INVENTION

A computer tomographic device and a method for correcting the radiationhardening is known from DE 100 51 462 A1. The known device has an x-raysource and an x-ray detector that together rotate around an object to beexamined. The projection images taken by the x-ray detector are appliedto an evaluation unit that corrects the radiation hardening. To do thisthe evaluation unit performs a post-reconstructive correction procedure.As part of the post-reconstructive correction procedure, the evaluationunit first reconstructs approximate volumetric images from the object tobe examined from the uncorrected projection images. The term volumetricimage in this case, and in the following, means both three-dimensionalvolumetric views and two-dimensional section images. A reprojection isthen performed, with only those pixels being used in the volumetricimage whose the image value is above a specified threshold value andthat are interpreted as materials to be distinguished from soft tissue.These materials can, for example, be bones or a contrast medium. Thelimitation to specific pixels enables the computing expense for thereprojection to be reduced.

With conventional computer tomography, a constant voltage is used forall projection directions, except for small fluctuations due to thegenerator for the tube voltage. The tube voltage is in this casepreferably chosen so that the radiation dose received by the detector isadequate for all projection directions and object thicknesses. If theobject to be examined is a patient, the patient under certaincircumstances is exposed to a dose of radiation that is greater thanwould be necessary to take the particular projection image.

Devices and methods have therefore been developed to reduce as far aspossible the radiation dose to which the patient is exposed. A deviceand a method of this kind are, for example, known from U.S. Pat. No.6,222,907 B1. With the known device and known method, the parameters ofthe x-ray tube are controlled corresponding to the beam path through theobject being examined.

The application areas for the known device and known method areradiography and fluoroscopy.

In recent times, the C-arch device for rotational angiography has beencontinuously improved. In particular, the mechanical stability of theC-arch has been increased, thus enabling approximate rotation about anisocenter. Together with the use of area detectors with an increaseddynamic compared with x-ray image amplifiers, this enables a computertomography volumetric reconstruction.

SUMMARY OF THE INVENTION

Starting from this prior art, the object of the invention is to providea device for computer tomography with an optimized radiation dose andgood image quality. The object of the invention is also to provide amethod for the reconstruction of volumetric images from projectionimages.

These objects are achieved by a device and a method with the features ofthe independent claims. Advantageous embodiments and developments aregiven in associated dependent claims.

The device is especially characterized in that the radiation source usedtransmits radiation with different energy distributions in variousprojection directions depending on the absorption characteristics of theobject to be examined, by adapting at least one operating parameter. Theevaluation unit supplied with the value used at a specific projectiondirection reads, from a data memory, a correction value allocated to thevalue of the operating parameter and thus corrects the radiationhardening on the relevant projection image.

Accordingly, for a method for reconstructing volumetric images, anevaluation unit is supplied with at least one operating parametertogether with projection image data, that is characteristic of theenergy distribution of the radiation used to take the projection images.Furthermore, correction values for radiation correction relative to thevalue of the operating parameters, stored in a data memory, are read bythe evaluation unit and the projection images are thus corrected withrespect to radiation hardening.

Because the operating parameters of the radiation source determine theenergy of the emitted radiation, the energy distribution of theradiation transmitted by the radiation source at known operatingparameters is also known. It is thus possible to determine in advancethe correction values for various values of the operating parameter,with which the radiation hardening can be corrected. The radiationhardening can thus be corrected in real time even with large amounts ofdata.

With a preferred form of embodiment, the radiation source is an x-raysource and the operating parameter the tube voltage of the x-ray source.Then, by means of the value of the tube voltage, the energydistribution, for a known material composition of the anode, of thex-ray photons emitted from the anode is known.

With a further preferred form of embodiment, the evaluation unitperforms what is called a water correction in that the evaluation unitdetermines, at a specific image value, a correction value stored in adata memory and relative to both the image value and the tube voltage.In this case it assumed for simplification that the attenuation of theradiation is caused by water-equivalent material.

Furthermore, the evaluation unit can also perform a post-reconstructivecorrection for radiation hardening relative to the tube voltage. To doso, the evaluation unit generates a three-dimensional object model,differentiated according to absorption characteristics, and allocates tothe image values object data records derived in each case from theobject model. Furthermore, the evaluation unit reads out from a datamemory the correction values allocated to the object data records andthe tube voltage, and thus performs the correction of the radiationhardening.

To reduce the computing expense, the evaluation unit preferably performsthe correction of the radiation hardening with a spatial resolution thatis less that the spatial resolution of the projection images. This isgenerally sufficient because the artifacts in the reconstructedvolumetric images induced by the radiation hardening generally have lowspatial frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the invention are given in thefollowing description, in which exemplary embodiments of the inventionare explained in detail using the accompanying drawings. These are asfollows:

FIG. 1 A perspective view of a computer tomography unit with a C-arch;

FIG. 2 A diagram showing the change in tube voltage of an x-ray sourcecompared with the projection angle;

FIG. 3 A diagram showing the spectra of an x-ray radiation emitted by atungsten anode at different tube voltages;

FIG. 4 A diagram showing the effective spectra ascertained by an x-raydetector at different tube voltages;

FIG. 5 A diagram showing the spectra resulting when twenty centimetersof water are x-rayed;

FIG. 6 The relationship of the attenuation coefficients of variousmaterials to the energy of x-ray photons;

FIG. 7 An illustration of the relationship of the mass attenuationcoefficients of various materials to the energy of x-ray photons;

FIG. 8 A diagram showing the relationship between projection valuessimulating passage through water and the path length for various tubevoltages;

FIG. 9 A cross-section through an object to be examined and a flat imagedetector for illustrating the material-selective reprojection;

FIG. 10 A block diagram for illustrating the sequence of a first orderrevaluation correction, known as the water correction;

FIG. 11 A block diagram to illustrate the sequence of a second orderiterative multispectral revaluation correction;

FIG. 12 A block diagram showing details of the second order iterativemultispectral hardening correction from FIG. 11;

FIG. 13 The representation of the reconstruction of an object, with thereconstruction being performed assuming constant tube voltage and usingprojection images that have been taken using variable tube voltage;

FIG. 14 The representation of the reconstruction of the object from FIG.13, taking account of voltage changes; and

FIG. 15 A representation of a differentiation image of the images shownin FIGS. 13 and 14.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a perspective view of an x-ray system 1 that can be usedfor rotational angiograph y. The x-ray system 1 enables thecomputer-tomographic volumetric reconstruction of the inner structure ofa patient 2. The x-ray system 1 includes an x-ray tube 3 and a detector4, that detects the x-ray radiation transmitted from the x-ray tube 3.On the way to the detector 4 the x-ray radiation passes through thepatient 2 so that the detector 4 takes projection images of the patient2. The detector 4 is preferably a digital area detector.

The x-ray tube 3 and detector 4 are mounted on a C-arch 5 that issecured by a mounting 6. The C-arch 5 is supported in the mounting 6 insuch a way that it can move in a circumferential direction 7. Themounting 6 is fitted to a stand 8 so that it can rotate about a rotaryaxis 9. The stand 8 is secured to a floor mounting 10 that enables thestand 8 to move.

When the x-ray system 1 is operating, the C-arch 5 rotates about therotary axis 9 and thus passes around a patient couch 11, on which thepatient 2 is supported.

The detector 4 is connected to an evaluation unit 12 that calculates avolumetric image of the inner structure of the patient 2 from theprojection images taken by the detector. The volumetric image can, forexample, be displayed on a monitor 13. Connected to the evaluation unit12 are mainly input devices 14 by means of which the x-ray system 1 canbe controlled.

In the case of conventional devices for high-speed computer tomography,the x-ray detector and the x-ray radiation source rotate around theobject to be examined at high speed in a fixed frame. Compared withthis, the x-ray tube 3 and detector 4 on the x-ray system 1 moverelatively slowly. Control of the tube voltage U matched to thedimensions of the object to be examined therefore appears relativelyeasy to accomplish.

FIG. 2 shows a voltage curve 15 of the typical pattern of the tubevoltage U for a rotational angiograph image of the heart. In this case,the C-arch 5 is, for example, moving over an angular range of 200degrees from the left anterior oblique position 100 to the rightanterior oblique position 100. During the movement of the C-arch 5, 200projection images are, for example, taken. To be able to compensate forthe different attenuation at different projection angles φ theparameters of the tube voltage, x-ray pulse width and tube current aredynamically matched during the rotation of the C-arch 5. For thoraxrotational images, the tube voltage U can vary completely in the rangebetween approximately 70 kV for anterior posterior radiation and 125 kVfor lateral imaging through the shoulder area.

Voltage-Dependent Radiation Hardening

The radiation of x-ray tube 3 is also polychromatic. The energy spectrumof the photons emitted as braking radiation at the anode depends mainlyon the applied tube voltage U, with which the electrons can beaccelerated from the cathode to the anode. At a tube voltage U, it isusually a high voltage in the kV range. The maximum photon energy isthenE _(max)(U)=U(keV/kV)=eU,with kilo electron volts (keV) usually being used as the unit of energy.Some typical emission spectra Q_(U)(E) for various voltages are shown inFIG. 3, in the emission spectra 16, 17 and 18 the pattern of theemission spectrum Q_(U)(E) in each case is shown at a tube voltage of 60kV, 90 kV and 120 kV. It should be noted that the anode of the x-raytube 3 is manufactured of tungsten and the radiation emitted from theanode is internally filtered through a 2.5 mm thick wall of aluminum.

However, the emission spectrum alone does not determine the imaging, butalso the transparency of the spectral filters usedW(E)=exp(−μ(E)T)with energy-dependent attenuation coefficient μ(E) and thickness T Thespectral response sensitivity η_(D)(E) of the detector is alsodeterminant for the imaging.

The resulting effective standard spectral distributions S_(U)(E) aretherefore defined by:S _(U)(E)=Q _(U)(E)W(E)η_(D)(E)/c _(U)  (#1)with the standard factor:

c_(U) = ∫₀^(eU)Q_(U)(E)W(E)η_(D)(E) 𝕕E,  ⇒ ∫₀^(eU)S_(U)(E) 𝕕E = 1.

Examples of effective spectral distributions S_(U)(E) are shown in FIG.4, where various resulting spectral distributions S_(U)(E) thatoriginate from emission spectra 16 to 18 when considering additionalfilters and the detector response sensitivity of the detector 4 arerecorded. In particular, the emission spectra 16, 17 and 18 each lead tospectral distribution 19, 20 and 21. For the case shown in FIG. 4, afilter of copper 0.3 mm thick was used and the detector was a CsIscintillator detector 0.55 mm thick with a density of 3.6 grams percubic cm.

Furthermore, during the penetration through matter the number oflow-energy photons is reduced more severely by absorption or scatterthan the number of high-energy photons, which leads to a radiationhardening depending on the material and path length. For example, thedominance of photons of higher energies in the resulting spectraldistribution S_(U) ^(R)(E) can be seen in FIG. 5. In FIG. 5, theresulting spectral distributions 22, 23 and 24 originating from theresulting spectral distributions 19, 20 and 21 during the passagethrough 20 cm of water are shown. A comparison with FIG. 4 clearly showsthat the resulting spectral distributions 19, 20 and 21 from FIG. 4 havebeen attenuated at the low-energy end when passing through 20 cm ofwater.

This phenomenon of radiation hardening occurs with objects made ofhomogenous material. With a cylindrical cross-section of water, forexample, with a radiation passage transverse to the longitudinal axisthe radiation hardening at the edge is less than in the area of thecenter of the cylinder where the radiation has to cover a long paththrough the cylinder.

However, the theory of reconstruction of volumetric images presumesmonochromatic radiation. Ignoring polychromacity leads, for example, tosomething called the cupping effect after the reconstruction, i.e. thereconstructed attenuation coefficient (gray value) reduces continuouslyfrom the edge inwards. This effect can be relatively easily correctedfor materials of a lower atomic number, that are similar to water, suchas soft tissue, fat and many plastics. The expression water correctionor first order hardening correction is used.

Furthermore, the radiation hardening is intensified by the presence ofmaterials with high atomic numbers, particularly by bones, contrastmedia or metal implants. Local density distortions occur after thereconstruction even after water correction, particularly bar orshadow-type artifacts, for example between heavily absorbent bonestructures. Such second order hardening artifacts 2 can reach anintensity of 10 to approx 100 HU (Hounsfield unit, corresponding to 0.1percent of the attenuation coefficient of water). The cause isultimately the energy dependency of the attenuation coefficients formaterials with a higher atomic number that deviates strongly from water.The correction of this effect is referred to in the following as secondorder hardening correction.

The dependency of the attenuation coefficient on the photon energy isshown in FIGS. 6 and 7 for various materials. FIG. 6 shows thedependence of the linear attenuation coefficient μ on the photon energy,whereas FIG. 7 shows the dependency of the mass attenuation coefficientμ/ρ. An attenuation curve 25 represents the attenuation through bonytissue. The attenuation curve 25 for bony tissue is distinctly above anattenuation curve 26 for soft tissue, above an attenuation curve 27 forfatty tissue, an attenuation curve 28 for Plexiglas and an attenuationcurve 29 for water. What is striking in FIG. 6 is that the attenuationcurve 26 for soft tissue is almost exactly the same as the attenuationcurve 29 for water. The mass attenuation coefficient shown in FIG. 7shows that the differences between bony tissue on the one hand and softtissue, fatty tissue and water on the other is even more distinct. Inthis case, the attenuation curve 25 for bone is clearly raised above theother attenuation curves 26 to 29, that lie comparatively closetogether.

Multispectral Water Correction: Preconstructive First Order RadiationHardening Correction

For simplicity, when considering water correction or first orderhardening correction the attenuation of an x-ray photon beam in theobject to be examined, that is usually a patient 2, is caused solely bywater-equivalent material. In this case water equivalence means that itis assumed that the energy dependency of the mass attenuationcoefficient (μ/ρ)(E) is identical to water and differences are due onlyto local differences in density. Accordingly, muscle tissue, blood oralso bony tissue is treated as water with a higher density (ρ>1 g/cm³⁾)

We now consider a measuring beam that penetrates the object to beexamined. Let the coordinate along its path be x and the local (linear)energy-dependent attenuation co-efficientμ(x,E)=ρ(x)α)(x,E),with the mass attenuation coefficient being shortened with α:α(x,E)=μ(x,E)/ρ(x).

The polychromatic logarithmic CT projection value for the measuring beamunder consideration is then

$\begin{matrix}\begin{matrix}{\overset{\sim}{p} = {- {\log\left( {\int_{0}^{eU}{{\exp\left( {- {\int{{\mu\left( {x,E} \right)}{\mathbb{d}x}}}} \right)}S_{U}\ {\mathbb{d}E}}} \right)}}} \\{= {- {\log\left( {\int_{0}^{eU}{{\exp\left( {- {\int{{\rho(x)}\alpha\;\left( {x,E} \right){\mathbb{d}x}}}} \right)}S_{U}\ {\mathbb{d}E}}} \right)}}}\end{matrix} & ({\# 2})\end{matrix}$with the measuring beam belonging to a projection number j, recorded ata tube voltage U=U_(j).

For this purpose, an equivalent water density b_(U)=b_(U)({tilde over(p)}) is determined in the following manner: let α_(W)(E) be theenergy-dependent mass attenuation coefficient of water, then thepolychromatic logarithmic projection value for a measuring beam with avoltage-dependent spectral distribution S_(U)(E), that is attenuatedalong a path length (coverage density) b in water (ρ=1 g/cm³) isdetermined as:

$\begin{matrix}{{f_{U}(b)} = {- {\log\left( {\int_{0}^{eU}{{\exp\left( {{- b}\;{\alpha_{W}(E)}} \right)}S_{U}\ {\mathbb{d}E}}} \right)}}} & ({\# 3})\end{matrix}$

This function can be calculated in advance for every voltage U or alsoexperimentally determined. In FIG. 8, the functions f_(U) are shown asprojection value curves 30 and 31 for the tube voltage relative tovarious voltage values. The projection value curve 30 shows therelationship between the polychromatic logarithmic projection value{tilde over (p)} depending on the path length b at a tube voltage of 70kV, and a projection value curve 31 shows the relationship between thepolychromatic logarithmic projection value {tilde over (p)} and the pathlength b at a tube voltage of 100 kV. The projection value curves 30 and31 rise monotonously with b and can be inverted. This preferably takesplace numerically, for example by means of an inverse interpolation.

For each measured value {tilde over (p)} in accordance with equation(#2) an equivalent water density b_(U)=b_(U)({tilde over (p)})=b can bedetermined so that {tilde over (p)}=f_(U)(b) applies in accordance withequation (#3), i.e. by inversion of equation (#3):b _(U) =f _(U) ⁻¹({tilde over (p)})  (#4)with b_(U) it is then possible to convert to the correspondingprojection value, that ideally would have been measured at amonochromatic spectrum with photons with only a single reference energyE₀. With b_(U) according to equation (#4) the corrected water-equivalentmonochromatic logarithmic project value resultsp ^(korr(0)=α) _(W)(E ₀)b _(U)=α_(W)(E ₀)f _(U) ⁻¹({tilde over (p)})=F_(U)({tilde over (p)})  #5)

In FIG. 8 a projection value curve 32 represents the pattern of theequivalent monochromatic logarithmic projection value p^(korr(0)) at aphoton energy E₀ of 40 kV.

The water correction can be illustrated using FIG. 8. With the measuredprojection value {tilde over (p)}, the associated b_(U) is sought usingthe projection value curve 30 or 31 matching the tube voltage. With thevalue for b_(U), the corrected monochromatic projection p^(korr(0)) canthen be sought by using the projection value curve 32.

It should be noted that in fact the conversion {tilde over(p)}→p^(korr(0)) depends on the voltage U. With the homogenous material,water, and the fixed specified path b, a constant path length b_(U)=bis, however, obtained from the inversion of the equation (#4) and aconstant monochromatic projection value p^(korr(0)) from the equation(#5), that in each case is independent of U.

It should also be noted that the right-hand sides of equations (#2) and(#3) are identical if the measuring beam penetrates a thickness b inwater. Then in the equation (#2) we get b=∫ρ(x)dx and α(x,E)=α_(W)(E)

Multispectral, Material-Selective Post-Reconstructive HardeningCorrection: Second Order Hardening Correction

Following the illustration of the first order hardening correction, thatis used directly on the projection data, a description of amultispectral, material-selective, second order hardening correction isnow described using FIG. 9. The second order hardening correction isbased on an iterative post-reconstructive correction approach, wherebythe physical process of the spectral hardening is remodeled using analready reconstructed, but not yet adequately corrected, volumetricimage 33. A material-selective segmentation is applied to the existingvolumetric image 33, which is usually a three-dimensional volumetricdimension image 33, by means of threshold criteria. FIG. 9 shows thesegmentation of the volumetric image 33. For simplicity, FIG. 4 showsonly the cross-section through the three-dimensional volumetric image33. The volumetric image 33 contains structure data of a patient 2, theouter contours of whom are shown by an ellipse in FIG. 9. Within thepatient 2 is, in addition to soft tissue 34, also bony tissue 35.Furthermore, vessels filled with contrast media or metal implants canalso be present. The volumetric image 33 is made up of individualvolumetric elements 36, known as voxels. The individual voxels areallocated to the categories soft tissue 34 or bony tissue 35 and toother categories as appropriate, depending on the gray values. Thevolumetric image 33 is then projected on pixel 37 of the detector 4. Indoing so, the mass coverage in grams per square centimeter for the softtissue 34 and bony tissue 35 along a measuring beam 38 allocated to theparticular pixel 37 is determined.

From the reprojection after the segmentation we then get, for eachindividual measuring beam 38, a value tuple for the coverage thicknesswith a density*path length unit in g/cm² of the various segmentedmaterial along the measuring beam 38 through the object volume.

The following explanations are, without restricting the generality,limited for simplicity to two materials with coverage thicknesses b_(W)and b_(k). By access to tables, generally followed by interpolation, acorrection factor is then allocated to the value pair (b_(W), b_(K)) forconversion of polychromatic projection data, disturbed by the hardeningeffect, into monochromatic projection data.

The multiparameter correction Table C, that is broken down into finediscreet steps relative to b_(W) and b_(K) and still depends on the tubevoltage U, can then be calculated in advance as follows before taking animage using the x-ray system 1, or if necessary also determined bymeasurements or adapted:C _(U)(b _(W) ,b _(K))=g ⁽⁰⁾(b _(W) ,b _(K) ,E ₀)/g _(U)(b _(W) ,b_(K),)  (#6)

In this case, g⁽⁰⁾ and g_(U) are the logarithmic mono- and polychromaticprojection values, defined by

$\begin{matrix}{{g^{(0)}\left( {b_{W},b_{K},E_{0}} \right)} = {{b_{W}{\alpha_{W}\left( E_{0} \right)}} + {b_{K}{\alpha_{K}\left( E_{0} \right)}}}} & ({\# 7}) \\\begin{matrix}{{g_{U}\left( {b_{W},b_{K}} \right)} = {- \log}} \\{\left( {\int_{0}^{eU}{{\exp\left( {{{- b_{W}}{\alpha_{W}(E)}} - {b_{K}{\alpha_{K}(E)}}} \right)}S_{U}\ {\mathbb{d}E}}} \right)}\end{matrix} & ({\# 8})\end{matrix}$

The comparison with equation (#3) shows that the following applies:f _(U)(b)=g _(U)(b,0)  (#9)

The hardening correction of the polychromatic measured projection data{tilde over (p)} then takes place by multiplication with a correctionfactor C_(U)p ^(korr) =C _(U)(b _(W) ,b _(K)){tilde over (p)}  (#10)or by additionp ^(korr) ={tilde over (p)}+δp ⁽¹⁾  (#11)with the correction projection dataδp ⁽¹⁾=(C _(U)(b _(W) ,b _(K))−1){tilde over (p)}  (#12)

It is noted that the corrections depend on the voltage U=U_(j) used inthe particular projection No. j. The corrected projection data or thecorrection projection data is used for a new volumetric imagereconstruction. The correction cycle can then be iteratively repeatedwith a new segmentation, with a new determination of material-specificcoverages b_(W)′,b_(K)′ by segmented reprojection, new correction inaccordance with equations (#10) and (#11)-(# 12) and with a newreconstruction.

Two-Stage Correction: Multispectral First and Second Order HardeningCorrection

It is pointed out that for the actual implementation in the x-ray system1 the correction (#11), {tilde over (p)}→p^(korr), is not performed inone step, but instead the water correction is carried out first. Thisoperates directly on the projection data and requires no reprojection.Only then is the deviation from the water correction, as a second ordercorrection, corrected. The segmented reprojection is then required forthis:First order correction: {tilde over (p)}→p^(korr(0)) according to (#5)Second order correction: p^(korr(0))→p^(korr)=p^(korr(0))+δp⁽²⁾  (#13)

$\begin{matrix}{{\delta\; p^{(2)}} = {\left( {{C_{U}^{(2)}\left( {b_{W},b_{K}} \right)} - 1} \right)p^{{korr}{(0)}}}} & ({\# 14}) \\{{C_{U}^{(2)}\left( {b_{W},b_{K}} \right)} = {{C_{U}\left( {b_{W},b_{K}} \right)}\frac{\overset{\sim}{p}}{p^{{korr}{(0)}}}}} & ({\# 15})\end{matrix}$

The corrections depend, as mentioned, on the voltage U=U_(j) used in theparticular projection No. j. The correction procedure can be iterativelycontinued.

Reduction of the Computing Expense of the Post-ReconstructiveCorrections

There are various methods of keeping the computing expense low. In DE100 51 462 the fact that the non-water-similar hardening materials witha higher atomic number, for example, bones, contrast media or metalusually have only a fraction of pixels 37 or voxels 36 is utilized byclever data organization.

Furthermore, it is possible to subject only correction projection data,corresponding to δp⁽¹⁾ or δp⁽²⁾, to a new volumetric imagereconstruction, in order to calculate a correction volumetric image andonly then superimpose it by addition to the uncorrected volumetricimage. This essentially uses the linearity of the image reconstructionbecause the linearity enables the sequence of addition andreconstruction to be switched.

Both methods of expense reduction can be combined.

In the following, a detailed description of the performance of thehardening correction is described with the aid of FIGS. 10 to 12. Thehardening correction taking place in the evaluation unit 12 can beimplemented both in the software and in the hardware. In the following,block diagrams that reflect the sequence are described and alsopseudocode is given.

Multispectral Water Correction

FIG. 10 shows the sequence of a water correction carried out by theevaluation unit 12.

First, a data acquisition 39, that leads to projection image data 40, isperformed with the aid of the detector 4. The projection image data 40also contains the particular tube voltage U used of the x-ray tube 3.Using the correction table 41 applicable for the tube voltage U in whichthe corrected projection values are entered relative to the measuredprojection values, a multispectral correction 42 of the beam revaluationis carried out. The correction 42 depends on the actual tube voltage Uof the x-ray tube 3. Using the corrected projection image data, an imagereconstruction 43 is then carried out, leading to a volumetric image 44with the radiation hardening due to water or body parts of the patient2, that have similar absorption properties to water, being corrected.

In the following, the process of water correction is described again bypseudocode.

When doing so, it is assumed that the (two-parameter) water correctionfamily of tables F_(U)(p) for the voltage range U_(min)≦U≦U_(max), usedfor system control and calculated in advance with suitablediscretization U=U_(n)=U_(min)+(n−1)ΔU, n=1,2, . . . , is available (forexample ΔU=5 kV).

The pseudocode is then:

for each projection direction j=1,N

with a projection angle φ_(j)=φ₀+(j−1)Δφ and tube voltage U_(j):

-   load table F_(U)( ) with U=U_(j) or interpolated table from F_(U)( )    and F_(U′)( ) with U=U_(n)≦U_(j)<U′=U_(n+1);-   read projection image {tilde over (p)}=({tilde over (p)}_(kl)),    whereby k,l are pixels indices of the projection image for    projection No. j;-   for each projection image pixel (k,l):-   water correction according to equation (#5) using table F_(U)( ):    {tilde over (p)}_(kl)→F_(U)({tilde over (p)}_(kl))-   for the corrected projection image p^(korr(0))=(F_(U)({tilde over    (p)}_(kl))):-   image reconstruction updating (additive superimposition in the    reconstruction volume)

It is pointed out that the image reconstruction is not limited to theFeldkamp algorithm, under certain circumstances with Parker weighting,at projection angles of less than 360 degrees. There are generalizationsthat are still back projections filtered from the type. Furthermore,every suitable reconstruction algorithm can, in principle, be used, forexample also a reconstruction method of the algebraic iterativereconstruction type.

Iterative, Multispectral, Second Order Hardening Correction

FIG. 11 is a flow diagram of an iterative, multispectral second ordercorrection of the beam hardening.

As for the water correction described using FIG. 10, the dataacquisition 39 leads to projection image data 40. Using the projectionimage data 40, an image reconstruction 45 was carried out that leads toan uncorrected volumetric image 46. The original projection image data40 and the uncorrected volumetric image 46 are each subjected tocoarsening 47 and 48, with the spatial resolution of the originalprojection image data 40 and uncorrected volumetric image 46 beingreduced. Using the data obtained in this way and the values for the tubevoltage U, a multispectral correction 49 of the radiation hardening isthen carried out, with the relevant allocated tube voltage U being takeninto account for the projection images.

By means of a succeeding refinement 50 of the correction volume imagesupplied from the correction 49, a correction volumetric image 51 isgenerated that is added to the uncorrected volumetric image 46 and acorrected volumetric image 52 thus results. As part of the refinement50, the spatial resolution of the correction volumetric image isincreased by interpolation corresponding to the spatial resolution ofthe uncorrected volumetric image 43.

In principle, the coarsening 47 and 48 and refining 50 steps can beomitted. This does, however, lead to a higher computing cost.

FIG. 12 shows the details of the correction 49 from FIG. 11. Thecoarsening 47 of the uncorrected projection image data 40 leads tocoarsened projection image data 53 and the coarsening 48 to a coarseneduncorrected volumetric image 54. A segmentation 55 in accordance withFIG. 9 is carried out using the coarsened uncorrected volumetric image54 and this is followed by a reprojection 56 that produces the masscoverage data 57, for example (b_(K), b_(W)). Depending on the tubevoltage U and mass coverage data 57, a correction table 58 is consultedthat contains the correction values C_(U)(b_(K), b_(W)) relative to thetube voltage U for conversion of polychromatic projection values intomonochromatic projection values. With this correction data from thecorrection table 58, a correction algorithm 59 is used that processesthe coarsened projection image data 53 and from this generates estimatedcorrection projection image data 60. Using the estimated correctionimage data 60, a reconstruction 61 is performed that leads to acorrection image 62 with less spatial resolution. A subsequent question63 is then asked to determine whether the correction image 62 hassubstantially changed in the last iteration step. If a substantialchange is present, the correction image 62 is added to the coarseneduncorrected volumetric image 54 and the segmentation 55 is performedagain. This is followed by a new reprojection 56 to generate improvedmass coverage data 57, followed by consultation of the correction table58 and performance of the correction algorithm 59 again, that leads toimproved, estimated correction projection image data 60. Thereconstruction 61 can then be repeated, so that an improved correctionimage 62 results.

If the correction image 62 has not substantially changed, the refining50 is carried out, leading to the correction volumetric image 51 withthe original spatial resolution.

The process of a second order hardening correction again usingpseudocode is described in the following.

It is again assumed that the (three or more parameter) family ofhardening correction tables C_(U)( ) according to equation (#6) isavailable, calculated in advance with suitable discretizing, for thevoltage range U_(min)≦U≦U_(max) used for the system control.

The pseudocode is then:

-   first volumetric image reconstruction with the aid of a standard    reconstruction-   volumetric image segmentation with material-selective threshold    values-   calculate the material-selective, hardening corrective volumetric    image as follows:-   for each projection direction j=1,N with a projection angle    φ_(j)=φ₀+(j−1)Δφ and tube voltage U_(j):-   load multiparameter, hardening correction table C_(U) with U=U_(j)    or interpolated table from C_(U) and C_(U′) with    U=U_(n)≦U_(j)<U′=U_(n+1);-   read projection image {tilde over (p)}=({tilde over (p)}_(kl)) with    k,l being pixel indices for the projection image for projection No.    j-   for each projection image pixel (k,l) and the measuring beam    striking this pixel:-   segmented (material selective) reprojection from which coverage    thicknesses b_(W),b_(K), . . . result-   hardening correction projection value according to equation (#12)    using the look-up table C_(U): {tilde over (p)}_(k,l)→p⁽¹⁾ _(kl)-   for the correction projection image δp⁽¹⁾=(δp⁽¹⁾ _(kl)): filtered    reprojection-   image reconstruction updating (additive superimposition in the    reconstruction volume)-   adding the hardening correction volumetric image as a    superimposition to the standard reconstruction volumetric image-   Iteration (optional): Repeat steps 1 * to 3*

Simulation Calculations

The method described here was tested in the simulation calculations.During the simulation calculations, a heavily simplified femur phantomwith low contrast inserts was used. FIG. 13 shows a cross-sectionthrough a reconstructed volumetric image that was taken using variablevoltage, but with the hardening correction having been performedassuming a constant voltage. FIG. 14 shows the same cross-section thatresults if the variability of the tube voltage is taken into accountwhen correcting the hardening in accordance with the method describedhere.

FIG. 15 then shows the differential image of the cross-section imagesfrom FIGS. 13 and 14. The errors reach approximately +/−20 HU in thesoft tissue.

The method described here and the x-ray system 1 described here has anumber of advantages.

With the x-ray system 1, the dose of the x-ray radiation can beminimized. By correcting the voltage dependent multispectral radiationhardening, the image quality is improved at the same time. Thissubstantially increases the quantitative accuracy of the reconstructedvolumetric images. Hardening artifacts are largely eliminated.

This means that it is then possible to consider the use of the methoddescribed here also in conjunction with conventional computer tomographydevices that have a fixed frame in which the x-ray source and the x-raydetector rotate.

It is pointed out that the method described here can be realized usingsoftware or with the aid of hardware. It is also pointed out that theterm evaluation unit is to be understood as being functional. Theevaluation unit does not necessarily have to be formed by a physicalunit but instead the function of an evaluation unit can also beperformed by several physical units.

It should finally be pointed out that with the exemplary embodimentdescribed here the tube voltage of the x-ray tube has been used to varythe energy distribution of the x-ray radiation. It is also conceivableto vary other operating parameters of the x-ray system 1. For example,the energy distribution of the x-ray radiation can also be varied byusing filters. In this case, the multiparameter correction table C mustalso be calculated relative to the additional operating parameters.Other operating parameters that influence the energy distribution of thex-ray radiation can be taken into account for other x-ray sources thatare used instead of the x-ray tube.

1. A computer tomography medical examination device, comprising: aradiation source for x-raying an object being medically examined from aprojection direction with a radiation energy associated to theprojection direction; a detector for detecting a radiation from theradiation source and recording a projection image of the object; a datamemory for storing a predetermined correction value; and an evaluationunit connected downstream of the detector for correcting a radiationhardening of the projection image, wherein an operating parameter of theradiation source is applied to determine the radiation energy associatedto the projection direction which depends on an absorptioncharacteristics of the object, wherein the evaluation unit is providedwith the operating parameter and reads the predetermined correctionvalue allocated to the operating parameter and corrects a radiationhardening on the projection image.
 2. The device as claimed in claim 1,wherein the radiation source is an x-ray tube and the detector is anx-ray detector.
 3. The device as claimed in claim 1, wherein a variableof the operating parameter is an x-ray tube voltage.
 4. The device asclaimed in claim 1, wherein the evaluation unit reads the predeterminedcorrection value relative to the x-ray tube voltage from the data memoryand carries out a water correction on the projection image by thepredetermined correction value.
 5. The device as claimed in claim 1,wherein the evaluation unit performs a post-reconstructive correction ofan hardening on the projection image that is caused by an attenuationwhich is different than a water-equivalent material in the object. 6.The device as claimed in claim 5, wherein the post-reconstructivecorrection of the hardening of the projection image is carried outiteratively by the evaluation unit.
 7. The device as claimed in claim 5,wherein the post-reconstructive correction of the hardening of theprojection image is carried out by the evaluation unit with a spatialresolution that is less than a spatial resolution of the projectionimage.
 8. The device as claimed in claim 1, wherein an object modeldifferentiated according to the absorption characteristics of the objectis determined by the evaluation unit from the projection image, whereinan object data record derived from the object model is allocated by theevaluation unit to a pixel of the projection image, wherein thecorrection value for the projection image is determined by theevaluation unit using the object data record and the x-ray tube voltage.9. The device as claimed in claim 1, wherein the object is a live animalor human patient.
 10. A method for correcting a radiation hardening on aprojection image of an object being medically examined, comprising:x-raying the object by a radiation source from a projection directionwith a radiation energy associated to the projection direction;detecting a radiation from the radiation source and recording aprojection image of the object by a detector; storing a predeterminedcorrection value in a data memory; correcting the radiation hardening ofthe projection image by an evaluation unit connected downstream of thedetector; and providing a corrected projection image, wherein anoperating parameter of the radiation source is applied to determine theradiation energy associated to the projection direction which depends onan absorption characteristics of the object, wherein the evaluation unitis provided with the operating parameter and reads the predeterminedcorrection value allocated to the operating parameter and corrects aradiation hardening on the projection image.
 11. The method as claimedin claim 10, wherein the radiation source is an x-ray tube and thedetector is an x-ray detector and the projection image is generated bythe x-ray tube and the x-ray detector.
 12. The method as claimed inclaim 10, wherein the tube voltage is used as a variable of theoperating parameter.
 13. The method as claimed in claim 10, wherein awater correction on the projection image is carried out by theevaluation unit by reading a predetermined correction value from thedata memory relative to the tube voltage applied.
 14. The method asclaimed in claim 10, wherein a post-reconstructive correction of theradiation hardening on the projection image is performed by theevaluation unit relative to the tube voltage, the hardening caused by amaterial having a different attenuating effect than a water-equivalentmaterial.
 15. The method as claimed in claim 14, wherein thepost-reconstructive correction of the hardening is performed iterativelyby the evaluation unit.
 16. The method as claimed in claim 14, whereinthe post-reconstructive correction of the hardening is performed with aspatial resolution that is less than a spatial resolution of the projectimage data.
 17. The method as claimed in claim 10, wherein athree-dimensional object model differentiated according to an absorptioncharacteristics of the object is determined by the evaluation unit fromthe projection image, wherein an object data record is derived from theobject model and is allocated to the project image, wherein the objectdata record and the tube voltage are used to determine a correctionvalue for the projection image from the data memory.
 18. The method asclaimed in claim 10, wherein the object is a live animal or humanpatient.